In pressurized water nuclear reactors, the core of the reactor is constituted by very long, square-sectioned fuel assemblies constituted by a bundle of rods containing the fuel material and disposed side by side, vertically and in contact at their side faces.
The transverse section of the core constituted by the juxtaposition of the square sections of the various assemblies is shaped like an irregular polygon whose perimeter has many steps.
The whole of the reactor core is held inside partitioning, in contact with the outwardly directed faces of the peripheral assemblies, over the whole height of these assemblies.
The core and its partitioning are also surrounded by a cylindrical shell, termed core casing, which provides a space, between the partitioning and its inner surface, inside which horizontal reinforcing plates are positioned ensuring that the partitioning, which is itself constituted by flat plates almost as long as the height of the core, is assembled and kept in position.
The reinforcing plates are pierced by openings allowing cooling water to circulate in the space between the partitioning and the core casing.
It is necessary in practice to cool the partitioning by the calculation of water over its outer surface and this cooling water can be introduced at the upper part of this space via water inlet orifices provided in the core casing.
As it circulates, the pressurized cooling water of the reactor enters the core via its lower part, passes through the assemblies vertically from bottom to top and is collected by the hot branches of the primary circuit at the upper part of the core. On its return, the cooling water from the partitioning runs through the space between the partitioning and the core casing, vertically from top to bottom before combining with the water entering the core at the lower part thereof. The loss of head of the pressurized water when it runs through the interior of the assemblies, depending on the height of the core, produces a difference in pressure, at the upper part of the core, between the water for cooling the core and the water for cooling the partitioning. In the case of cooling the partitioning by descending current, this difference in pressure is of the order of 2 bars.
The vertical plates constituting the partitioning are simply juxtaposed and assembled at right angles by screws.
In some pressurized water nuclear reactors currently operating, the assembly between some partitions has only a small number of screws, so that a leakage space can occur between the corresponding partitioning elements.
Because of the difference in pressure existing, at least at the upper part of the partitioning, between the core region and the peripheral region between the partitioning and the core casing, pressurized jets of water directed from the exterior to the interior of the core create regions of turbulence in the neighbouring assemblies and cause vibrations in the rods adjacent to these leakage regions which can cause them to deteriorate in the long run.
To solve this problem, an attempt has been made to reduce the leakage spaces between such partitions, inside nuclear reactors, by hammering the joints between partitions responsible for these phenomena.
Such a hammering operation must be carried out during a reactor shutdown, under water, with special tooling since the reactor materials are contaminated after use of this reactor. Before the hammering operation, the joints with too great play must be identified and then, after hammering, a check must be made that the play is sufficiently small to practically eliminate the occurrence of pressurized jets through the partitioning.
These operations are therefore relatively complex and take a long time. In addition, when the reactor is in service, the joints can be displaced again so that the pressurized jets are very likely to reappear some time after the hammering operation.